Generally speaking, drillers and well completion engineers can exploit hydrocarbon reservoirs more effectively when they are better informed about the conditions in and around the well bore. Such information typically includes characteristics of the earth formations traversed by the borehole, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD).
In wireline logging, a probe or “sonde” is lowered into the borehole after some or the entire well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole. The direct electrical connection between the surface and the sonde provides a relatively large bandwidth for conveying logging information.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated. While LWD techniques allow more contemporaneous, and often more accurate, formation measurements, it is difficult to establish and maintain a direct electrical connection in an LWD environment. Consequently, the logging data is often stored in internal memory until the sensing instruments are retrieved to the surface where the logging data can be downloaded. Alternatively, LWD communication channels such as mud pulse signaling, through-wall acoustic signaling, and/or low frequency electromagnetic wave signaling can be employed to communicate the logging data to the surface.
A wide variety of wireline and LWD sensing instruments are available. However, the traditional “triple-combo” log is adequate for most wells, and in any event, it is run in nearly every well even when other logs are collected. The traditional “triple-combo” log includes four logs: a natural gamma ray log, a resistivity log, a density log, and a neutron porosity log.
The natural gamma ray log measures the natural radioactivity of the formation surrounding the borehole. This radioactivity provides an indication of the presence of certain minerals in the formation and it can be helpful in distinguishing reservoir formations from surrounding beds.
The resistivity log measures the resistivity of the formation surrounding the borehole. In porous formations, this resistivity is dominated by the conductivity of the fluids in the pore space. Accordingly, this log is helpful in distinguishing hydrocarbons from groundwater.
The density log measures the scattering of gamma rays emitted into the formation from the borehole. The intensity of scattered gamma rays is indicative of the density of electrons in the formation, which in turn is indicative of the density of the rock in the formation.
The neutron porosity log measures the energy loss rate or capture rate of neutrons emitted into the formation from the borehole. These rates are dominated by the concentration of hydrogen atoms in the formation. Because the presence of this hydrogen is primarily due to the water or hydrocarbon fluids in the pore space of the rock, the measured rates provide an indication of the porosity of the formation rock.
Ground water salinity is an important parameter that affects not only the accuracy of the neutron porosity measurement, but also affects the resistivity-log-based determination of water and hydrocarbon saturation. The neutron porosity measurement is adversely affected because the chlorine strongly absorbs neutrons, whereas the water saturation determination is adversely affected because the salinity determines the ground water's conductivity.
The standard techniques for determining ground water salinity are: (1) take a water sample from the well after it has been drilled, or (2) extrapolate from other wells in the region. Experience has shown this approach to be unreliable due to significant salinity variation within and between wells, particularly when fluid injection is employed for secondary recovery. In the latter case, the fluid injection causes substantial salinity variation laterally across the reservoir and vertically around the reservoir, making the standard techniques impractical or hopelessly inaccurate.